Lymphocytes play a dominant role in immune surveillance. In this capacity, they are responsible for beneficial effects to the organism such as destruction of tumors and development of resistence to infection. This recognition and response to foreign antigens can have deleterious effects, however, such as the rejection of transplanted organs. The lymphocytes which mediate cellular immune reactions such as graft injection are thymus-derived (T) lymphocytes; those which form circulating antibodies are bone marrow-derived (B) cells. Both classes of lymphocytes possess specific receptors for antigen. Moreover, these cells must collaborate in an, as yet, unknown manner in order to form antibodies to various antigens. This proposal is designed to study the chemical nature and physiological functions of the lymphocyte surface receptor for antigen. This will involve isolation and characterization of surface immunoglobulin (Ig) from T-cells and B-cells which was made possible by application of lactoperoxidase-catalyzed radioiodination to living lymphocytes. The binding specificity for antigen of surface Ig from specifically activated T-cells will be ascertained. Experiments will be performed to elucidate the biochemical mechanisms of lymphocyte activation initiated by binding of antigen to receptor. Furthermore, the role of cell surface Ig in collaboration between T-cells and B-cells will be studied. This research will include a detailed analysis of the structural and functional properties of lymphocyte surface proteins.